The present invention relates to an improved method of producing .epsilon.-caprolactam from cyclohexanone oxime in the gaseous phase using MFI catalysts on whose surface symmetrically arranged OH groups are present. The present invention also concerns such novel MFI catalysts on whose surface symmetrically arranged OH groups are present and to a method of producing them.
.epsilon.-Caprolactam is of great economic significance as the starting point for nylon-6-synthesis (Kirk-Other's Encyclopedia of Chemical Technology, (Third Edition), Volume 18, pages 425-436; this excerpt is entirely incorporated herein by reference). In the classic production method primarily used today, cyclohexanone oxime is converted on fuming sulfuric acid by the Beckmann rearrangement to .epsilon.-caprolactam (see, e.g., U.S. Pat. No. 4,968,793 which is incorporated by reference in its entirety). A disadvantage of this method is the use of ammonia which is necessary to neutralize the sulfuric acid and is associated with an accumulation of ammonium sulfate of up to 4.5 tons per ton caprolactam. In order to avoid this undesired byproduct, efforts have been undertaken since the middle of the 1960s to carry out the reaction by heterogeneous catalysis.
It is known that .epsilon.-caprolactam can be produced from cyclohexanone oxime in the gaseous phase on solid catalysts, e.g. on zeolites with MFI structure. The term MFI structure or topology is well known in the art (see, e.g., U.S. Pat. Nos. 5,374,747; 5,451,701; and 5,453,511, all of which are incorporated by reference in their entirety).
X and Y zeolites and mordenites in the H form or doped with rare earth metals or transition metals deactivate extremely rapidly. 5-Cyanopent-1-ene, cyclohexanone and cyclohexanol are formed increasingly as byproducts on these catalysts. Temperatures below 400.degree. C. and atmospheric pressure have been suggested as process parameters. Furthermore, non-polar solvents such as cyclohexane, benzene or toluene are supposed to be more advantageous than more polar solvents.
The partial blocking of the acidic centers of the HY zeolite with Na did not result in an improvement of the results (M. Burquet et al., An. Quim. Ser. A 81 (1985) 259; A. Aucejo et al., Appl. Catal, 22 (1986) 187 and A. Corma et al., Zeolites 11 (1991) 593). Rather, the authors determined that strongly acidic Bronsted acidic centers with pKa&lt;1.5 are necessary for the Beckmann rearrangement and that the selectivity-reducing byproduct 5-cyanopent-1-ene is formed both on the acidic centers and on the Na.sup.+ ions. Furthermore, the authors suspected that the deactivation of the zeolite catalyst takes place by means of basic byproducts such as aniline and methylpyridine and not by coke formation on the catalyst surface since the color of the catalyst is still al most white after the reaction.
In order to avoid the disadvantages of the rapid ageing of X and Y zeolite catalysts, pentasil zeolites (MFI structure) have been extensively examined.
Zeolites with MFI structure are also favorably viewed in EP 0,494,535 (corresponding to U.S. Pat. Nos. 5,212,302 and 5,403,801 which are incorporated by reference in their entirety) and EP 0,544,530 (corresponding to U.S. Pat. No. 5,304,643 which is incorporated by reference in its entirety). However, a high Si/M ratio of 5 to over 500 is required here, which the metal (M) can be Al, B, Ga, among others. Alcohols and ethers as well as water are cited as additive components and solvents. The maximum addition of water is approximately 0.06 to 2.5 moles per mole oxime.
The attempts at improving the service life of the catalyst also took the direction that the outer surface of a boron-MFI zeolite was covered with Na ions in order in this manner to neutralize the acidity on the outer surface (EP 0,086,543 (Jul. 5, 1986) Stamicarbon; corresponding to U.S. Pat. No. 4,472,516 which is incorporated by reference in its entirety). A mixture of cyclohexanone oxime, toluene, carbon dioxide and water in a molar ratio of 1:3:7:1 at 340.degree. C. is said to convert the cyclohexanone oxime quantitatively on this catalyst. The selectivity for .epsilon.-caprolactam is indicated as 58%. The service life of the catalyst treated with Na ions is approximately 15 hours and is more than three times as long as in the case of the untreated boron MFI zeolite. However, the byproduct 5-cyanopent-1-ene is also strongly formed on this catalyst and thus does not constitute a satisfactory result.